All the above advice is good. My advice is DON'T DO IT!!!!!
I organized a family reunion cruise last summer and boy was it a nightmare. "Can you book it the following week? Little Johnny has a soccer match that week."/ "We may not be able to go that week because in laws are thinking of visiting" / "Should we book the two kids in our room or get them a separate room?" / "Why are you booking that hotel? I can get a better rate at this hotel on Travelocity?" blah, blah, blah!
I would advise you to pick a week, pick a cruise, and stick with it. Some people are going to be upset because that week or that cruise doesn't suit them, but so be it!
And then find a travel agent and refer everyone to the TA!